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unordered_map::insert

Inserts an element or a range of elements into an unordered_map.

// (1) single element
pair<iterator, bool> insert(
    const value_type& Val
); 

// (2) single element, perfect forwarded
template<class ValTy>
pair<iterator, bool> insert(
    ValTy&& Val
);

// (3) single element with hint
iterator insert(
    const_iterator Where,
    const value_type& Val
); 

// (4) single element, perfect forwarded, with hint
template<class ValTy>
iterator insert(
    const_iterator Where,
    ValTy&& Val
);

// (5) range 
template<class InputIterator> 
void insert(
    InputIterator First,
    InputIterator Last
); 

// (6) initializer list
void insert(
    initializer_list<value_type> IList
);

Parameter

Description

Val

The value of an element to be inserted into the unordered_map unless it already contains an element whose key is equivalently ordered.

Where

The place to start searching for the correct point of insertion.

ValTy

Template parameter that specifies the argument type that the unordered_map can use to construct an element of value_type, and perfect-forwards Val as an argument.

First

The position of the first element to be copied.

Last

The position just beyond the last element to be copied.

InputIterator

Template function argument that meets the requirements of an input iterator that points to elements of a type that can be used to construct value_type objects.

IList

The initializer_list from which to copy the elements.

The single-element member functions, (1) and (2), return a pair whose bool component is true if an insertion was made, and false if the unordered_map already contained an element whose key had an equivalent value in the ordering. The iterator component of the return-value pair points to the newly inserted element if the bool component is true, or to the existing element if the bool component is false.

The single-element-with-hint member functions, (3) and (4), return an iterator that points to the position where the new element was inserted into the unordered_map or, if an element with an equivalent key already exists, to the existing element.

No iterators, pointers, or references are invalidated by this function.

During the insertion of just one element, if an exception is thrown but does not occur in the container's hash function, the container's state is not modified. If the exception is thrown in the hash function, the result is undefined. During the insertion of multiple elements, if an exception is thrown, the container is left in an unspecified but valid state.

To access the iterator component of a pair pr that's returned by the single-element member functions, use pr.first; to dereference the iterator within the returned pair, use *pr.first, giving you an element. To access the bool component, use pr.second. For an example, see the sample code later in this article.

The value_type of a container is a typedef that belongs to the container, and for map, map<K, V>::value_type is pair<const K, V>. The value of an element is an ordered pair in which the first component is equal to the key value and the second component is equal to the data value of the element.

The range member function (5) inserts the sequence of element values into an unordered_map that corresponds to each element addressed by an iterator in the range [First, Last); therefore, Last does not get inserted. The container member function end() refers to the position just after the last element in the container—for example, the statement m.insert(v.begin(), v.end()); attempts to insert all elements of v into m. Only elements that have unique values in the range are inserted; duplicates are ignored. To observe which elements are rejected, use the single-element versions of insert.

The initializer list member function (6) uses an initializer_list to copy elements into the unordered_map.

For insertion of an element constructed in place—that is, no copy or move operations are performed—see unordered_map::emplace and unordered_map::emplace_hint.

For a code example, see map::insert.

Header: <unordered_map>

Namespace: std

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